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Taken by Surprise
by Peter Tyson
October 17, 1998

It happened during a dive at Lobster Rock late yesterday afternoon. Bob Cranston and Michele Hall had gone off to look for a batfish, one of the ocean's stranger creatures, while Peter Kragh, Lance Milbrand, and I dove on the sandy flats north of the Rock, hoping to see silvertip sharks. We soon found them—sleek, beautiful juveniles no more than three feet long. Pestered by rainbow runners, a silvery fish known to chafe the sides of these sharks, the elusive silvertips came near only when we held as still as possible. The water was a bit murky, and they never approached closer than about 15 feet, which lent them an enigmatic mien.

As we were heading back towards Lobster Rock, we were stopped by a swarm of black triggerfish. Also known as black durgon, this fish is a striking blue- black, with distinctive white lines tracing its tail end at the base of the dorsal and anal fins. It's a joy to see even a single black triggerfish, but here there were hundreds, swirling in an underwater tornado right on the bottom. So dense you couldn't see through it, the swarm was animated by bursts of movement as six or eight triggerfish at once spun off in tight pursuit of a single one of their kind. Was it males going after a female? Were they courting, or mating, or doing something entirely different? I didn't know, so I just watched.

I was so entranced by the tornado that it was a few minutes before I looked overhead. Then I saw that the tornado of triggerfish was only one part of a great storm of them. Their hovering black silhouettes filled the sky above us like a plague of aquatic locusts. Spinning around, I could see no end to them. At one point they temporarily dispersed, shooting off as one toward the safety of the bottom when a school of fast-moving mullet snapper swept through. But soon they reappeared, and the puzzling ritual continued. With dusk rapidly approaching, we reluctantly left them.

Back on the boat, there was excited speculation. Kragh thought they were probably mating, which seemed a reasonable enough explanation, though no one had seen that behavior before. Earlier in the day, the film crew had witnessed a similar black triggerfish swarm at Manuelita. As baffled as we were, Howard Hall tried to film it, but as soon as he turned the movie lights on the swarm, it broke up. Mark Thurlow told me, "I've never seen anything like it."

The waters around Cocos are full of such surprises. For most divers, it's stunning enough just to see sharks on every single dive, as we do, or hundreds of hammerheads. Kragh told me diving in the Red Sea he felt lucky to encounter a single ray; here you see marbled rays at every turn, occasional spotted eagle rays, and, if you're lucky, mobula and Pacific manta rays. The sheer piscine biomass at Cocos Island rivals any place in the world.

But then there are those unexpected surprises. As an inexperienced diver, at least compared to my companions aboard the Undersea Hunter, I enjoyed eye- openers on every dive. Within minutes of reaching the bottom on my inaugural dive here three weeks ago, Kragh and I spied a tiger snake eel sliding sinuously over the sandy seafloor. Every eel I'd ever seen before hid in crevices. On subsequent dives: a huge, all-yellow grouper, standing out in a twilit sea like a streetlamp; a shoebox-sized slipper lobster "like a prehistoric tank" (Mark Conlin); in the final minutes of a dive at Lobster Rock, a green sea turtle, paddling off into the blue.

Even the Halls have been taken by surprise, which is saying something. They've seen everything and filmed it to boot. For them, such surprises have come not in sightings but in unusual behaviors. A green sea turtle is nothing to them, but a green sea turtle feeding on a branch of leaves from a rainforest tree is. (They filmed this unexpected behavior earlier this year.) No one had ever seen a mantis shrimp at Cocos Island before the Halls found one near Yglesias Bay, nor had anyone ever seen one quite as large (about 14 inches long). But that's not why they filmed it. They filmed it for its remarkable attack strategy (see the mantis shrimp video clip). Even the courtship of the marbled rays was an unplanned bonus. Since the rays usually court in summer, the Halls had given up on them this late in the season—and then they went ahead and did it.

Even the second mass ritual that we ran into on yesterday's dive had been a surprise for the Halls. After leaving the swarm of triggerfish, the three of us made our way in the gathering twilight back towards the jumble of boulders at the base of Lobster Rock. On the way, Kragh surfaced and reboarded the skiff, which inexplicably motored off, leaving Milbrand and I alone. Happy to have a little more time, I kept swimming. Then I noticed Milbrand pointing: In front of us lay a vast white cloud, suspended in the water column about 20 feet below the surface. What was it? Ahead I saw Cranston, back from his search for the batfish. I swam towards him, but was soon engulfed in the snowy nimbus.

Instantly I had that uneasy feeling you get when your plane flies into a cloud and you lose all visibility. I couldn't see my hand in front of my face. I had a sudden thought that I might get lost—a ridiculous one since Milbrand and Cranston were less than 20 feet away. After a minute or so the cloud began to dissipate, and then there was Cranston, ushering me towards a darkened reef below.

There, on a twilit ledge, I discovered the source of the mysterious cloud. Hundreds of cream-colored fish with bold vertical stripes teemed above the ledge. Every half minute or so, a cluster of the fish would start quivering with apparent excitement and then rocket towards the surface, u-turning after 10 or 15 feet and leaving behind a cloud of whiteness. Other clusters, perhaps inspired by the instigators' obvious agitation, performed the same feat virtually simultaneously. It was synchronized swimming as only nature can do it. The whole effect, as Michele Hall commented later, was like a burst of fireworks.

It was convict tangs spawning, I learned later. So-named for the black lines that paint their sides like prison stripes, the convict tangs, the Halls told me, congregate on this ledge every evening. During the massive mating dance that follows, males and females release sperm and eggs together. The cloud that swallowed me? Millions of potential tangs. For the Halls as for me, the spawning of the convict tangs was not in the original script. Like the tornado of triggerfish, it was an unexpected surprise.

We leave tomorrow night for the mainland, a trip of 30-some hours, so look for the next and final dispatch on Tuesday, the 20th.

Peter Tyson is Online Producer of NOVA.

Photo: ©Michele Hall.

Hammerheads or Bust (Sept. 23)
Get Used To It (Sept. 25)
Nature Reigns at Cocos (Sept. 27)
The PIG and the Process (Sept. 29)
Hammerheads Sighted (Oct. 1)
Assault on Cocos (Oct. 3)
The Director's Cut (Oct. 5)
Swimming with Whitetip Reef Sharks (Oct. 7)
The Magnificent Seven (Oct. 9)
The Search for Lake Cocos (Oct. 11)
Courtship of the Marbled Rays (Oct. 13)
Of Booby and Beebe (Oct. 15)
Taken by Surprise (Oct. 17)
"This is Cocos, This is Cool" (Oct. 19)

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